1. Field of the Invention
The invention relates to stabilized, encapsulated fuels and methods for the encapsulation of fuels. More particularly, the invention relates to the encapsulation of water reactive, hydrogen gas generating fuels.
2. Description of the Related Art
Similar to batteries, fuel cells function to produce electricity through chemical reactions. Rather than storing reactants as batteries do, fuel cells are operated by continuously supplying reactants to the cell. In a typical fuel cell, hydrogen gas acts as one reactant and oxygen as the other, with the two reacting at electrodes to form water molecules and releasing energy in the form of direct current electricity. This direct current electricity may then be converted into an alternating current. The apparatus and process may produce electricity continuously as long as hydrogen and oxygen are provided. While oxygen may either be stored or provided from the air, it is generally necessary to generate hydrogen gas from other compounds through controlled chemical reactions rather than storing hydrogen, because storing hydrogen gas requires that it either be compressed or cryogenically cooled. As fuel cell technology evolves, so do the means by which hydrogen gas is generated for application with fuel cells.
Currently, there are various methods which are known and employed for generating hydrogen gas. One method is by a process known as reformation in which fossil fuels are broken down into their hydrogen and carbon products. However, this system is undesirable in the long term because it is dependent upon a non-renewable resource. Another means of generating hydrogen gas is by reversibly adsorbing and releasing hydrogen gas from metal hydrides or alloys through heating. While this method is useful, it is not preferred because the metal hydrides are typically very heavy, expensive and only release small quantities of hydrogen. Yet another means by which hydrogen gas is generated is through reactive chemical hydrides. This process involves chemically generating hydrogen gas from dry, highly reactive solids by reacting them with liquid water or acids. Chemicals especially suitable for this process are lithium hydride, calcium hydride, lithium aluminum hydride, sodium borohydride and combinations thereof, each of which is capable of releasing plentiful quantities of hydrogen. Compared to the above methods, the use of reactive chemical hydrides is highly desirable in the art, particularly for generating power for use by small, portable electronic devices, such as cellular phones. However, it also has its disadvantages. For example, it has been found that the reaction products from the chemical hydride and liquid water typically form a cake or pasty substance which interferes with further reaction of the reactive chemical with the liquid water or acid. Furthermore, the reaction of chemical hydrides with liquid are difficult to control, and typically generally results in the production of much more hydrogen gas than needed to power such small electronic devices.
In order to combat this problem, methods have been introduced wherein a hydrogen fuel can be reacted with only gaseous water vapor, instead of liquid water. For example, U.S. Pat. No. 4,155,712 teaches an apparatus for generating hydrogen by the reaction of a metal hydride with water vapor, wherein a water reservoir is provided and the metal hydride is housed in a separate fuel chamber. A liquid water source is provided in a water chamber, and water molecules from the liquid water source are introduced into the fuel chamber by diffusing through a porous membrane. U.S. Pat. No. 4,261,955 also teaches an apparatus for generating gas by the reaction of a metal hydride fuel with water vapor, wherein water vapor from a liquid water reservoir is introduced into a fuel chamber through a pair of spaced porous hydrophobic membranes. In each of these designs, an elaborate power generator system is required in order to regulate the quantity of water vapor that reacts with the chemical fuel and to regulate the reaction rate of water vapor with the chemical fuel.
It would be desirable in the art to provide a method in which the rate of reaction between water molecules and a water reactive chemical fuel can be regulated independently of the apparatus containing the chemical fuel. Further, it has been discovered that hydrogen gas generators that operate based on reactions between a chemical hydride and water can explode or generate hydrogen at an excessive rate if they are damaged and the chemical hydride is exposed to liquid water. Accordingly, it would be further desirable in the art to provide a secure fuel system for a hydrogen gas generator wherein the hydrogen generation rate is limited such that the generator will not explode or rapidly generate hydrogen gas if the generator is damaged.
The present invention provides a solution for this need in the art. The invention provides a stabilized, or passivated, chemical hydride which is encapsulated in a water vapor permeable, liquid water impermeable material, such as Gore-Tex®. Alternately, the chemical hydride may be coated with an oil or rubber substance to passivate the surface of the chemical fuel and prevent liquid water permeation while allowing water vapor permeation.